Oxygen monitoring and apparatus

ABSTRACT

Apparatus or systems which employ luminescence quenching to produce an oxygen concentration indicative signal. Components of such systems include: (1) an airway adapter, sampling cell, or the like having a sensor which is excited into luminescence with the luminescence decaying in a manner reflecting the concentration of oxygen in gases flowing through the airway adapter or other flow device; (2) a transducer which has a light source for exciting a luminescable composition in the sensor into luminescence and a light sensitive detector for converting energy emitted from the luminescing composition as that composition is quenched into an electrical signal indicative of oxygen concentration in the gases being monitored; and (3) subsystems for maintaining the sensor temperature constant and for processing the signal generated by the light sensitive detector. Sensors for systems of the character just described, methods of fabricating those sensors, and methods for installing the sensors in the flow device.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the monitoring of oxygen concentrationand, more particularly, to novel, improved methods and apparatus formonitoring the concentration of oxygen in respiratory and other gasesand to components of and controls for apparatus as just characterized.

In another aspect, the present invention relates to methods ofmanufacturing airway adapters designed for use in on-airway applicationsof the invention.

In a third aspect, the present invention relates to novel sensors whichinclude an oxygen quenchable luminescable compound and methods formanufacturing sensors of the character.

BACKGROUND OF THE INVENTION

The most common cause of anesthetic and ventilator related mortality andmorbidity is inadequate delivery of oxygen to a patient's tissues.Therefore, the monitoring of static inspired oxygen concentration haslong been a safety standard of practice to ensure detection of hypoxicgas delivery to patients undergoing surgery and to those on mechanicalventilators and receiving supplemental oxygen therapy. However,monitoring the static inspired fraction of inhaled oxygen does notalways guarantee adequate oxygen delivery to the tissues because it isthe alveolar oxygen concentration that eventually enriches the blooddelivered to the cells.

It is this alveolar gas phase that is interfaced with pulmonaryperfusion which, in turn, is principally responsible for controllingarterial blood gas levels. It is very important that the clinician knowthe blood gas levels (partial pressure) of oxygen (pO₂) and carbondioxide (pCO₂) as well as the blood pH. Blood gas levels are used as anindication of incipient respiratory failure and in optimizing thesettings on ventilators. In addition, blood gas levels can detectlife-threatening changes in an anesthetized patient undergoing surgery.

The traditional method for obtaining arterial blood gas values is highlyinvasive. A sample of arterial blood is carefully extracted and thepartial pressure of the gases is measured, using a blood gas analyzer.Unfortunately, arterial puncture has inherent limitations: (1) arterialpuncture requires a skilled health care provider and it carries asignificant degree of patient discomfort and risk, (2) handling theblood is a potential health hazard to the health care provider, (3)significant delays are often encountered before results are obtained,and (4) measurements can only be made intermittently.

Non-invasive methods for estimating blood gas levels are available. Suchmethods include the use of capnography (CO₂ analysis). These methodsemploy fast gas analyzers at the patient's airway and give a graphicportrayal of breath-by-breath gas concentrations and, therefore, canmeasure the peak exhaled (end tidal) concentrations of the respectiverespired gases. Although gradients can occur between the actual arterialblood gas levels and the end tidal values, this type of monitoring isoften used as a first order approximation of the arterial blood gasvalues.

Other techniques have been utilized for assessing patient blood gaslevels with mixed results. Transcutaneous sensors measure tissue pO₂ andpCO₂ diffused through the heated skin surface. This type of sensor has anumber of practical limitations including a slow speed of response anddifficulty of use.

Pulse oximetry is widely used to measure the percentage of hemoglobinthat is saturated with oxygen. Unfortunately, it does not measure theamount of dissolved oxygen present nor the amount of oxygen.carried bythe blood when hemoglobin levels are reduced. This is important becauselow hemoglobin levels are found when there is a significant blood lossor when there is insufficient red blood cell information. In addition,pulse oximeter readings are specific to the point of contact, which istypically the finger or ear lobe, and may not reflect the oxygen levelof vital organs during conditions such as shock or hypothermia.

Oxygraphy measures the approximate concentration of oxygen in the vitalorgans on a breath-by-breath basis and can quickly detect imminenthypoxemia due to decreasing alveolar oxygen concentration. For example,during hypoventilation, end tidal oxygen concentration changes morerapidly than does end tidal carbon dioxide. During the same conditions,pulse oximetry takes considerably longer to respond. Fast oxygenanalysis (oxygraphy) can also readily detect inadvertent administrationof hypoxic gas mixtures.

Oxygraphy reflects the balance of alveolar O₂ available duringinspiration minus the O₂ uptake secondary to pulmonary perfusion. Anincreasing difference between inspiratory and end tidal oxygen values isa rapid indicator of a supply/demand imbalance which could be a resultof changes in ventilation, diffusion, perfusion and/or metabolism of thepatient. This imbalance must be quickly corrected because failure tomeet oxygen demand is the most common cause of organ failure, cardiacarrest, and brain damage. Oxygraphy provides the earliest warning of thedevelopment of an impending hypoxic episode.

Oxygraphy has also been shown to be effective in diagnosing hypoglycemicor septic shock, air embolism, hyperthermia, excessive PEEP, CPPRefficacy, and even cardiac arrest. During anesthesia, oxygraphy isuseful in providing a routine monitor of preoxygenation(denitrogenation). It especially contributes to patient safety bydetecting human errors, equipment failures, disconnections,misconnections, anesthesia overdoses, and esophageal intubations.

Combining the breath-by-breath analysis of oxygen with the measurementof airway flow/volume as outlined in U.S. Pat. Nos. 5,347,843 and5,379,650 gives another dimension to the clinical utility of oxygraphy.This combination parameter, known as oxygen consumption (VO₂), providesan excellent overall patient status indicator. Adequate cardia output,oxygen delivery, and metabolic activity are all confirmed by oxygenconsumption because all of these physiological processes are requiredfor oxygen consumption to take place. Oxygen consumption is also usefulin predicting ventilator weaning success.

A metabolic measurement (calorimetry) includes determination of apatient's energy requirements (in calories per day) and respiratoryquotient (RQ). Interest in the measurement of caloric requirements hasclosely paralleled the development of nutritional support. For example,the ability to intravenously provide all the necessary nutrition tocritically ill patients has only been accomplished within the last 25years. Along with the realization that we need to feed patients, hascome the need to know how much to feed them, what kind of nutrients(carbohydrates, lipids, protein) to feed them, and in what ratio thenutrients need to be supplied. The only true way to measure the caloricrequirements of patients and to provide a non-invasive qualityassessment of their response to nutrition is with indirect calorimetry.Airway O₂ consumption and CO₂ production can be measured non-invasivelyand provide a basis for the computations needed for a measurement ofindirect calorimetry, a direct measurement of the metabolic status ofthe patient, and the patients' respiratory quotient.

With the above clinical need in mind, it is important to ensure thatclinicians have the proper equipment to monitor breath-by-breath oxygen.While there are adequate devices for measuring static levels of oxygen,the measurement of breath-by-breath (fast) airway oxygen concentrationrequires more sophisticated instruments. Very few of these devices canbe directly attached to the patient airway. Instead, most require theuse of sampling lines to acquire the gas and send it to a remote sitefor analysis. Fast airway oxygen monitors are typically large, heavy,fragile instruments that consume considerable power. They must sampleairway gases via a small bore plastic tube (sidestream) and remotelydetect the oxygen gas as it passes from the airway to the sensor. Theproblems associated with this type of gas sampling are well known. Gasphysics dictates painstaking, careful measurements because water vaporconcentration pressure and temperature can vary within the patient'sairway and gas sample line. The presence of water and mucous createproblems for long term patency of the sample tube. Also, the sample lineacts like a low pass filter and affects the fidelity of the measurement.Finally, the pressure variable delay introduced by the sample linecreates difficulty in accurately synchronizing the airway flow andoxygen concentration signals required to calculate oxygen consumption.

On-airway (mainstream) monitoring of oxygen has the potential to solveall of the above problems, especially when breath by breath monitoringoxygen consumption measurements are to be made. However, most of theavailable fast oxygen sensors are simply too big, too heavy, toofragile, and/or otherwise not suited to be placed in line with apatient's breathing tube.

There are various other technologies which have been employed inmonitoring airway oxygen concentration. Some of the most widely used areelectrochemical sensors. These fall into two basic categories:polarographic cells and galvanic cells. These cells produce an electriccurrent proportional to the number of oxygen molecules which diffuseacross a membrane. The advantages of these types of sensors aresimplicity and low cost. The disadvantages include limited lifetime(chemistry depletes) and slow response (not breath-by-breath). In somecases, these cells have demonstrated sensitivity to certain anestheticagents, which introduces inaccuracies into the oxygen concentrationmeasurement. Generally, this type of sensor is too large to attach tothe patient airway.

There have been a few reported developments where electrochemical cellmembranes were improved to enable faster response. There are alsosilicon micromachined cells using the principle of “Back Cell”electrochemical technology. Their time response approaches 150 ms butthey appear to be subject to the typical problems of this type of cell(i.e., stability and calibration).

Another popular medical oxygen sensor is the paramagnetic type. Thissensor uses the strong magnetic property of oxygen as a sensingmechanism. There are two basic types of paramagnetic cells: static anddynamic. The static type is a dumbbell assembly suspended between thepoles of a permanent magnet. The magnetic forces of the surroundingoxygen molecules cause a torsional rotation of the dumbbell which can besensed optically and employed as a measure of oxygen concentration. Thedynamic type (see U.S. Pat. No. 4,633,705) uses a magneto-acousticapproach. This requires a gas sample and a reference gas that are mixedwithin an electromagnetic field. When the field is switched on and off,a pressure signal proportional to the oxygen content is generated. Thesignal can be detected by a differential microphone. The advantages ofthe paramagnetic sensor are good linearity and stability. The dynamictype is inherently faster responding that the static type. Both typesare subject to mechanical vibration, and the dynamic type has thedisadvantage of requiring a reference gas. Neither type is suitable foron-airway applications.

Zirconium oxide cells are frequently used in the automotive industry tomeasure oxygen concentration. The cell is constructed from a solidelectrolyte tube covered by platinum electrodes. When heated toapproximately 800 degrees C., a voltage proportional to the Icgarithm ofthe ratio between a sample gas and a reference gas is generated. Theadvantage of this sensor are wide dynamic range, very fast response, andsimplicity. The high cell temperature is clearly a disadvantage as ispower consumption. Also, the cell is degraded in the presence ofanesthetic agents. Clearly, this type of cell cannot be used on apatient airway.

Ultraviolet absorption uses the principle that oxygen exhibitsabsorption properties in the ultraviolet part of the electromagneticspectrum (about 147 nm). This technique has been used in several medicalapplications but has never been reduced to commercial viability. Thereare numerous technical difficulties which make this a difficulttechnique for on-airway applications.

Mass spectrometers spread ionized gas molecules into a detectablespectrum according to their mass-to-charge ratios and can accordingly beused to measure oxygen concentration. These instruments are generallylarge assemblies with ionizing magnets and high vacuum pumps. Theadvantages of mass spectrometers include high accuracy, multi-gasanalysis capability, and rapid response. The disadvantages include highcost, high power consumption, and large size. Mass spectrometers are notsuitable for on-airway applications.

Raman scattering spectrometers (as described in U.S. Pat. No. 4,784,486)can also be used to measure oxygen concentration. These devices respondto photons emitted by the collision of a photon with an oxygen molecule.A photon from a high-power laser loses energy to the oxygen molecule andis re-emitted at a lower energy and frequency. The number of photonsre-emitted at the oxygen scattering wavelength is proportional to thenumber of oxygen molecules present. Like mass spectrometers, Ramanspectrometers have multi gas analysis capability and rapid responsetime. Disadvantages include large size and power consumption. Therefore,Raman scattering photometers are not suitable for on-airwayapplications.

Visible light absorption spectrometers (as described in U.S. Pat. Nos.5,625,189 and 5,570,697) utilize semiconductor lasers that emit lightnear 760 nm, an area of the spectrum comprised of weak absorption linesfor oxygen. With sophisticated circuitry, the laser can be thermallyand/or electronically tuned to the appropriate absorption bands. Theamount of energy absorbed is proportional to the number of oxygenmolecules present. The advantages of this system are precision, fastresponse, and no consumable or moving parts. The disadvantages includesomewhat fragile optical components, sensitivity to ambient temperatureshifts, and a long gas sample path length. While there have beenattempts to utilize this technology in an on-airway configuration, nocommercially viable instruments have so far been available.

Luminescence quenching has also been proposed as a technique formeasuring oxygen concentration. In this approach a sensor contacted bythe gases being monitored is excited into luminescence. Thisluminescence is quenched by the oxygen in the monitored gases. The rateof quenching is related to the partial pressure of oxygen in themonitored gases, and that parameter can accordingly be used to providean indication of the oxygen in the monitored gases. However, nowhere inthe prior art are the problems addressed that require resolution for anoxygen concentration monitor employing luminescence quenching to be ofany practical value addressed. These include:photo-degradation-associated and other instabilities of the sensor, lowsignal level, noise leading to difficulties in assessing the decay ofsensor luminescence, acceptably fast response times, thermal drift ofthe sensor, reproducibility of the sensors, inaccuracies attributable tostray light reaching the data photodetector, and the need for lightweight, ruggedness, and low power consumption.

Consequently, there is an existent and continuing need for devices andmethods which can be used on-line to obtain a fast (i.e.,breath-by-breath), non-invasive measurement of oxygen concentration inrespiratory gases.

SUMMARY OF THE INVENTION

There have now been invented and disclosed herein certain new and novelmethods of, and devices for, monitoring oxygen concentration in gaseousmixtures. These novel devices differ from the majority of the oxygenmonitors described above in that they are compact, lightweight, andotherwise suited for on-airway mainstream monitoring of the oxygenconcentration in a person's respiratory gases. The methods andmonitoring devices disclosed herein utilize the fast (orbreath-by-breath) approach to oxygen concentration monitoring with thequenching of a luminescent dye being used in determining theconcentration of oxygen in the gases being monitored.

Fast (breath-by-breath) monitoring of end tidal oxygen is an importantdiagnostic tool because, as examples only:

1. It is a sensitive indicator of hypoventilation.

2. It aids in rapid dianosis of anesthetic/ventilation mishaps such as(a) inappropriate gas concentration, (b) apnea, and (c) breathingapparatus disconnects.

3. End tidal oxygen analysis reflects arterial oxygen concentration.

4. Inspired-expired oxygen concentration differences reflect adequacy ofalveolar ventilation. This is useful for patients undergoing ECMO(Extracaporeal Membrane Oxygenation) or nitric oxide therapies.

5. When combined with a volume flow device (e.g. a pneumotach), VO₂(oxygen consumption) can be determined. Oxygen consumption is a veryuseful parameter in determining (a) oxygen uptake during ventilation orexercise, (b) respiratory exchange ratio or RQ (respiratory quotient)and (c) general patient metabolic status.

The novel sensor devices of the present invention locate a luminescentchemical in the patient airway. Modulated visible light excites thechemical and causes it to luminesce. The lifetime of the luminescence isproportional to the amount of oxygen present. A transducer containing aphotodetector and associated electronic circuitry measures decay timeand relates the measured parameter to the ambient oxygen partialpressure.

The transducer device is small (<1 cubic inch), lightweight (less than 1ounce), and does not contain moving parts. It utilizes visible lightoptoelectronics and consumes minimal power (system power less than 2watts). The unit warms up in less than 30 seconds, which is advantageousin on-airway applications because of the need to take prompt remedialaction if a change occurs in a patient's condition reflected in a changein respiratory oxygen concentration. The assembly does not require anysignificant optical alignment and is very rugged (capable of beingdropped from 6 feet without effecting optical alignment or otherwisedamaging the device).

Yet another important advantage of the present invention is that itsprinciples can be employed to advantage in sidestream (sampling) typesystems as well as in mainstream systems. This is important because somegas analysis systems, such as anesthetic analyzers, employ sidestreamtechniques to acquire the gas sample.

A typical transducer unit is easy to calibrate, stable (±2 torr over 8hours at a 21 percent oxygen concentration), and has a high resolution(0.1 torr) and a wide measurement range (oxygen concentrations of 0 to100 percent). Response to changing oxygen concentrations is fast (<100ms for oxygen concentrations of 10-90 percent at flow rates ≈1 l/min).The transducer is not susceptible to interference from anestheticagents, water vapor, nitrous oxide, carbon dioxide, or other gases andvapors apt to be present in the environment in which the system is used.

The sensor comprises a polymeric membrane in which a luminescablecomposition such as a porphyrin dye is dispersed. The sensor membrane isthe mediator that brings about dye-oxygen interaction in a controlledfashion. In a functional sensor, the dye is dispersed in the polymericmembrane, and oxygen diffuses through the polymer. The characteristicsof the sensor are dependent upon the dye-polymer interaction andpermeability and the solubility of oxygen in the polymer. Suchcharacteristics include the sensitivity of response of the sensor tooxygen, the response time of the sensor to a change in oxygenconcentration, and the measured values of phosphorescence intensity anddecay time. Thus the composition and molecular weight of the polymerdetermines the sensor characteristics. Also, if the sensor is preparedby evaporation of a solution as described below, the filmcharacteristics depend on the solvent that is used and conditions duringcasting or evaporation. If the dye is separately doped into the filmfrom another solution, the solvent and conditions in the doping mediumalso affect the sensor characteristics. When the polymer film isprepared by polymerization of a monomer-or mixture, the sensorcharacteristics depend on the conditions of polymerization and suchresultant polymer characteristics as degree of crosslinking andmolecular weight.

The luminescent chemical sensor is not toxic to the patient and is apart of a consumable (i.e., disposable) airway adapter weighing lessthan 0.5 ounce. The sensor shelf life is greater than one year and theoperational life exceeds 100 hours. The cost of the consumable airwayadapter is minimal.

It is also important that the oxygen monitoring system of the presentinvention has sufficient accuracy (1.0%), precision (0.01%), andresponse time (<100 ms) to monitor breath-by-breath oxygenconcentrations. A related and important advantage of the presentinvention is that the sensor is not sensitive to other gases found inthe airway, including anesthetic agents, and is accordingly not excitedinto luminescence by those gases. The sensitivity of the sensor totemperature, flow rate, pressure and humidity change is well understood;and algorithms which provide compensation for any errors due to thesechanges are incorporated in the signal processing circuits of thedevice.

It is a further advantage that the sensor can be easily (and evenautomatically) calibrated to single point room air oxygen, which isimportant because of the lack of availability of calibration gases incertain settings. The device is so stable that recalibration is notrequired for at least eight hours.

One embodiment of the present invention employs a single light sourcefor exciting the luminescable composition of the sensor, a data detectoron which light propagated from the luminescing composition falls as thatluminescence is quenched by oxygen in the gases being monitored, and areference detector for calibrating the data detector.

A second embodiment of the present invention employs two sources oflight for exciting the sensor into luminescence and a single (data)photodetector. This arrangement has the advantage of eliminatingmeasurement errors attributable to differential drift between the dataand reference signal processing circuits.

Preferred embodiments of the visible light oxygen measurementtransducers disclosed herein employ a novel sensor heater arrangementand a proportionalintegrated-differential (PID) heater control systemfor keeping the oxygen concentration sensor of the transducer preciselyat a selected operating temperature. This is particularly significantbecause the oxygen measurement transducers disclosed herein employ asensor which involves the use of the diffusion of oxygen into aluminescable layer in measuring oxygen concentration. The rate ofdiffusion is temperature dependent. As a consequence, the measurement ofoxygen concentration becomes inaccurate unless the sensor temperature iskept constant.

In on-airway applications of the invention, the oxygen concentrationsensor takes the form of a thin film mounted in an airway adaptercasing; and the sensor heater includes a highly conductive thermalcapacitor for heating the sensor film. A novel assembly method insuresthat the sensor film is stretched over the thermal capacitor in theassembled airway adapter and that the thermal capacitor and sensor filmare therefore in intimate physical contact. This further promotes theprecision with which the sensor can be maintained at the selectedtemperature by guaranteeing a rapid transfer of heat between the thermalcapacitor and the film so that the film temperature cannot drift to anyappreciable extent from the selected operating temperature. This isreflected in an accurate oxygen concentration measurement.

In on-airway applications of the invention, the thermal capacitor in theairway adapter is heated by way of a floating, thermally conductiveheater component in the oxygen measurement transducer to which theairway adapter is removably assembled. The floating heater and thermalcapacitor are so configured that the heater snaps into firm physicalcontact with the capacitor as the airway adapter is assembled to thetransducer. This insures that there is intimate contact between, and anefficient transfer of heat from, the floating heater to the thermalcapacitor.

A thick film resistance heater can be used to heat the transducer'sfloating heater element. This element is preferably located on that sideof the floating heater opposite the side contacted by the airway adapterthermal capacitor along with a temperature sensing component of theheater control system. The temperature sensor is incorporated in the PIDcontrol system for the thick film heater.

The location of the oxygen concentration sensor in a replaceable, simplecomponent is an important feature of the present invention. This makesit possible to readily and inexpensively ensure that the system issterile with respect to each patient being monitored by replacing theairway adapter between patients, avoiding the nondesireability (andperhaps the inability) to sterilize that system component.

The provision of an airway adapter sensor and a separatesignal-producing transducer also has the practical advantage that ameasurement of oxygen concentration can be made without interruptingeither the ventilation of a patient, or any other procedure involvingthe use of the airway circuit. This is affected by installing the airwayadapter in the airway circuit. When the time comes to make oxygenmeasurements, all that is required is the transducer be coupled to theairway adapter already in place. The adapter includes a casing made froma thermally non-conductive polymer that defines a passage through whichthe gases to be analyzed flow. The airway adapter sensor is coupled to atransducer to generate a signal indicative of the oxygen concentrationin the gases flowing through the airway adapter. Another importantfeature of the invention insures that the airway adapter and transducerare assembled in the correct orientation and that the airway adapter andtransducer are securely assembled until deliberately separated by thesystem user.

The signals generated by the novel oxygen-measurement transducersdisclosed herein must be processed to remove noise and extract theluminescence decay time, which is the oxygen-sensitive parameter ofinterest. A lock-in amplifier is preferably employed for this purpose.The lock-in amplifier outputs a signal which has a phase anglecorresponding to the decay time of the excited, luminescent compositionin the oxygen concentration sensor. The lock-in detection circuitryrejects noise and those components of the photodetector-generated signalwhich are not indicative of oxygen concentration. This noise reductionalso allows a higher level of signal gain which, in turn, makes possibleenhanced measurement precision while decreasing the level of the visibleexcitation. This reduces instability from photoaging of the sensor,increasing accuracy and useable life. All of this processing, which canbe done with digital, analog, or hybrid methods, is fast enough for eventhe most demanding applications such as those requiring thebreath-by-breath monitoring of a human patient. Various pathologicalconditions result in a change of oxygen demand by the body. If adecrease of oxygen utilization by the body, for example, can be detectedon a breath-by-breath basis, timely and effective remedial steps can betaken to assist the patient.

In the novel oxygen measurement transducers of the present invention,the concentration of oxygen in the gases being monitored is reflected inthe quenching of an excited luminescent composition in the oxygenconcentration sensor by oxygen diffusing into the sensor matrix. Asource consisting of a light-emitting diode (LED) produces visibleexciting light which strikes the surface of the sensor film. Some of thelight is absorbed by the luminescent chemical dye in the film whereuponit produces luminescent light at a second, shifted wavelength. All lightdirected toward the photodetector can potentially result in a signal. Asuitable optical filter placed over the surface of the photodetectordiscriminates against all but the luminescent light, thereby ensuringthat the photodetector is producing a signal related to oxygenconcentration only. The goal of isolating the photodector from lightwhich is not indicative of oxygen concentration can be furthered by ageometric relationship of the light source and photodetector asestablished by the configuration of an optical platform on which thelight source and photodector are mounted. This geometric relationshipplaces the photodetector at a location away from the specular reflectionof the LED light off of the surface, further optimizing the ratio ofluminescent light to other, stray or reflected light that might reachthe detector.

The novel oxygen-sensitive sensors employed in the present inventioninclude a luminescent composition, uniformly distributed over, andembedded in, a thin, porous polymer matrix, a scheme which ensures afast sensor response time. Novel methods, disclosed herein, formanufacturing these sensors are simpler than those heretofore proposed,give more reproducible results, and allow the matrix to be fabricatedfrom a wide variety of polymers with desirable characteristics. In thesemethods, a solution of the selected luminescent dye is painted onto, orsoaked into, a porous polymeric membrane or sandwiched between twomembranes of the selected polymer. Due to the porous structure of thestarting polymers, the films or membranes have the advantage that themolecules are embedded within microns of the gas-polymer interface andhave fast response times. As the starting material is a thin polymericmembrane, batch processing of films of uniform composition andcharacteristics is facilitated.

As suggested above, on-airway monitoring is a particularly advantageousapplication of the present invention. The principles of the presentinvention can, nevertheless, be advantageously employed in other gasmonitoring techniques; notably, sidestream sampling, and can be used tomonitor the oxygen concentration of other than respiratory gases.

The objects, features, and advantages of the invention will be apparentto the reader from the forgoing discussion and the appended claims andas the ensuing detailed description of the invention proceeds inconjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generally pictorial view of an on-line system for monitoringthe oxygen concentration in a patient's breath; the system isconstructed in accord with and embodies the principles of the presentinvention;

FIG. 2 is an exploded view of an airway adapter and a complementarytransducer employed in the FIG. 1 system; the airway adapter has asensor for oxygen in respiratory gases flowing through the adapter andthe transducer acts with the airway adapter to provide a signalindicative of the concentration of the oxygen in the monitored gases;

FIG. 3 is an exploded view which includes a perspective of thetransducer and illustrates how the airway adapter is kept from beingincorrectly assembled to the transducer;

FIG. 4 is an exploded view of the transducer showing the two componentsof the transducer casing and an assembly which includes the opticalcomponents of the transducer and a platform to which those componentsare mounted;

FIG. 5 is a transverse section through the airway adapter takenprimarily to show the details of the oxygen sensor incorporated in theairway adapter and of a thermal capacitor included in the airway adapterto keep the temperature of the oxygen sensor constant;

FIG. 6 is a longitudinal section through the airway adapter and isprovided to show: the passage through the adapter for the gases beingmonitored, the oxygen concentration sensor and thermal capacitor, and awindow which transmits light to and from the oxygen sensor;

FIGS. 7-10 show the steps employed in installing the oxygenconcentration sensor and the thermal capacitor in the casing of theairway adapter;

FIG. 11 is a cross-sectional plan of the optical platform subassembly ofthe transducer; the optical components of the transducer are mounted tothe platform of this subassembly;

FIG. 12 is a front view of the optical platform subassembly;

FIG. 13 is a view of the transducer with one of the two casingcomponents removed and certain components sectioned to show the internalcomponents of and the optical paths in the transducer;

FIG. 14 is a generally pictorial view showing how a light source and adata detector in the transducer are so geometrically related by theplatform of the optical subassembly that unwanted light which mightaffect the accuracy of the oxygen concentration signal outputted fromthe transducer is kept from reaching the detector;

FIG. 15 is a fragmentary exploded view of the FIG. 2 system componentsshowing how the airway adapter fits the transducer;

FIG. 16 is a view similar to FIG. 15 but showing the airway adapterpartially installed in the transducer with the thermal capacitor in theairway adapter coming into conductive heat transfer relationship with acomplementary heater in the transducer;

FIG. 17 shows, in block diagram form, the operating system of the FIG. 1apparatus;

FIG. 18 is a block diagram of a lock-in amplifier circuit incorporatedin the FIG. 17 operating system; the lock-in amplifier is employed toisolate from accompanying noise that component of the signal produced bya data detector in the transducer which is actually indicative of theconcentration of oxygen in the gases being monitored;

FIG. 19 is a block diagram of a heater control incorporated in thetransducer; this control is employed in maintaining the sensor in theairway adapter at a constant operating temperature;

FIG. 20 is a block diagram showing how the signal propagated from thedata detector of the FIG. 4 transducer is converted to a signalindicative of the concentration of oxygen in the gases being monitored;

FIG. 21 is a block diagram showing two different protocols forprocessing the detector-generated signal after that signal has beenprocessed by the transducer electronics; one protocol is employed ifthere is a high concentration of oxygen in the gases being monitored,and the second protocol is employed if the oxygen concentration is low;

FIG. 22 is a view, similar to FIG. 13, of a second oxygen concentrationmonitoring transducer employing the principles of the present inventionand consisting of an airway adapter and a transducer with dual lightsources and a single detector;

FIG. 23 is a front view of an optical subassembly employed in the FIG.22 transducer and consisting of an optical platform to which the duallight sources and detector are mounted;

FIG. 24 is a section through FIG. 23 optical assembly takensubstantially along line 24—24 of FIG. 23;

FIG. 25 shows a sidestream sampling, oxygen concentration monitoringsystem employing the principles of the present invention;

FIG. 26 illustrates a nasal canula component of the exemplary FIG. 25system; and

FIG. 27 shows, in some detail, a sampling cell and a transducer of theFIG. 25 system; this figure also shows in block diagram for a signalprocessing and control unit of the system.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, FIG. 1 depicts oxygen concentrationmonitoring apparatus 30, which is constructed in accord with andembodies the principles of the present invention. The major componentsof apparatus 30 include an online assembly 32 of a transducer 34 and anairway adapter 36. The particular system 30 illustrated in FIG. 1, alsoincludes a hand held control and display unit 38 which is connected totransducer 34 by a conventional electrical cable 40.

In the particular application of the present invention illustrated inthe drawings, system 30 is employed to monitor the concentration ofoxygen in a patient's respiratory gases. To this end, airway adapter 36is connected in line between an endotracheal tube 42 inserted in thepatient's trachea and the plumbing 44 of a mechanical ventilator (notshown).

Airway adapter 36 and transducer 34 cooperate to produce an electricalsignal indicative of the oxygen concentration in the gases flowing fromendotracheal tube 42 through airway adapter 36 to plumbing 44. Thissignal is transmitted to unit 38 through cable 40 and converted to anumerical designation, which appears on the display array 46 of unit 38.

The just-described two-component systems meets the requirement thatmonitoring be accomplished without interrupting the flow of gasesthrough breathing circuit 44 or other patient connected flow circuit.Transducer 34 can be removed—for example, to facilitate or enable themovement of a patient—leaving airway adapter 36 in place to continue thevital flow of gases.

System 30 also has, in this regard, the advantage that there are noelectrical components in the airway adapter. Hence, there are nopotentially dangerous electrical connections to the airway adapter orexposure of a patient to electrical shock.

Referring now most specifically to FIGS. 2, 3, and 6, the exemplary,illustrated airway adapter 36 is a one-piece unit typically molded fromValox polycarbonate or a comparable polymer which is rugged and can bemolded to close tolerances. An opaque material is employed to keepambient light from reaching a light sensitive sensor component 47 of theairway adapter through the walls of the airway adapter. Such extraneouslight would adversely affect the accuracy of the oxygen concentrationreading which the system is designed to provide.

Airway adapter 36 has a generally parallelepipedal center section 48 andhollow, cylindrical end sections 50 and 52. Axially aligned passages 54,56, and 58 respectively found in airway adapter elements 50, 48, and 52define a flow passage 60 extending from end-to-end through airwayadapter 36.

As shown in FIGS. 2 and 6, end section 50 of airway adapter 36 isconfigured as a male connector, and end section 52 is configured as afemale connector. This allows the airway adapter to be connected intoconventional anesthetic and respiratory circuits.

As is perhaps best shown in FIGS. 5 and 6, apertures 62 and 64 alignedalong transverse axis 66 are formed in opposed side walls 68 and 70 ofairway adapter central element 48. An oxygen concentration sensorassembly 72 is mounted in aperture 62, and a window 74 is mounted inaperture 64 facing sensor 72 and on the opposite side of the sensor fromflow passage 60.

Sensor assembly 72 (see FIGS. 5, 6, and 10) is composed of sensor 47 anda thermal capacitor 78. Sensor 47 is a thin film which is stretched overand thereby in intimate contact with the thermal capacitor. As will bediscussed later, thermal capacitor 78 is employed to maintain sensor 47at a constant operating temperature and thereby eliminate inaccuraciesin oxygen concentration measurement attributable to variations in thetemperature of sensor 47.

Sensor 47 is made up of a thin, microporous, hydrophobic polymericmatrix with a luminescable composition disposed in the matrix. Thepreferred luminescable compositions are photostable, phosphorescentdyes, which absorb energy having a frequency between 300-700 nm, emitenergy with a frequency in the range of 500-1000 nm, and have aluminescence decay time in the range of 1-1000 microseconds.

Oxygen monitoring apparatus embodying the principles of the presentinvention operates on the principle that the luminescable composition ofsensor 47 can be excited into luminescence by a pulse of light of anappropriate frequency with that light being absorbed by the luminescablecomposition and re-emitted at a shifted wavelength over, typically, aperiod measured in microseconds. Oxygen in gases passing through theflow passage 60 of airway adapter 36 quenches the luminescingcomposition. The quenching of the composition is related to the oxygenconcentration of the gases flowing through airway adapter passage 60. Asthe oxygen concentration increases, the quenching of the excited stateof the composition does likewise; and the intensity and characteristicdecay time of the luminescence decreases. This quenching is a dynamicprocess with response of the sensor to a change in oxygen concentrationbeing sufficiently fast to allow monitoring of oxygen on abreath-by-breath basis. No chemical reactions occur in theexcitation/quenching cycle, so the luminescible composition is not usedup in the oxygen monitoring process.

Previous attempts to employ luminescence quenching in the measurement ofoxygen concentration have focused on increasing illumination of theoxygen sensor, thereby increasing the magnitude of response of thesensor to changes in the oxygen concentration of the gases beingmonitored. However, the high illumination level gives rise to rapidphoto-aging of the sensor film, limiting accuracy and stability. Systemsemploying the principles of the present invention, in contrast, decreasethe illumination level and use higher electronic gain of the detectorgenerated electrical signal, and selective noise reduction, reducing theinstability due to photo-aging of the sensor film to acceptable levels.

A large-area photodiode detector further facilitates the use ofdecreased illumination levels. The size of the LED beam can also beexpanded to fill a larger area of the sensor film, thereby lowering theintensity of illumination per unit area while leaving the total signalnearly unchanged. This along with slight aperturing of the LED beam andreduction of the LED duty cycle (pulse rate) easily leads to anorder-of-magnitude decrease in aging rate.

The presently preferred luminescable compositions are porphyrins.Porphyrins are stable organic ring structures that often include a metalatom. When the metal atom is platinum or palladium, the phosphorescencedecay time ranges from 10 microseconds to 1000 microseconds. This givesa high sensitivity to oxygen and allows fairly simple electronicdetection of the energy emitted by the excited composition.

Some of the synthetic porphryins are especially stable with respect tophotodegradation. The fluorinated porphyrins; e.g., the meso-tetraphenylporphines, are especially photostable. Luminescable compositions of thischaracter which can be employed to advantage in systems employing theprinciples of the present invention are: platinum meso-tetra(pentafluoro) phenyl porphine, palladium meso-tetra (pentafluro) phenylporphine, platinum meso-tetraphenyl prophine, and palladium meso-tetraphenyl prophine.

The sensor membrane (or matrix) is an important element of apparatusembodying the principles of the present invention because it bringsabout sensor composition-oxygen interaction in a controlled fashion. Theluminescable compositions are dispersed in the polymer as byevaporation, doping, or in situ polymerization. The characteristics ofthe sensor are dependent upon the composition-polymer interaction andthe permeability and solubility of oxygen in the polymeric matrix. Suchcharacteristics include the sensitivity of the sensor to oxygen, theresponse time of the sensor to a change in oxygen concentration, and themeasured values of phosphorescence (a luminescence) intensity and decaytime.

The composition and molecular weight of the polymer also determine thesensor characteristics. Furthermore, if the polymer film is prepared byevaporation of a solution, the film characteristics depend on thesolvent and the process conditions during casting or evaporation. If theluminescable composition is separately doped into the film, the solventand process conditions employed in the doping also affect the sensorcharacteristics. When the polymer film is prepared by polymerization ofa monomer or mixture, the sensor characteristics depend on theconditions of polymerization and such resultant polymer characteristicsas degree of crosslinking and molecular weight.

In short, a variety of not obvious factors must be taken into account inselecting the membrane material and in fabricating the membrane from theselected material.

The aforementioned process parameters enable a high degree ofengineering of sensor characteristics. At the same time, many variablesare controlled, resulting in the production of sensor films ofoptimized, uniform characteristics.

Many previous approaches to phosphorescence quenching have focused onsilicone polymers due to their high oxygen permeability. These polymers,however, suffer from low solubility for many phosphorescent organic ororganometallic compounds and low signals resulting from the lowluminescable composition content and high quenching although the highoxygen diffusion rates in silicone films gives a rise to short responsetime to changing oxygen concentration (e.g., less than 1 sec.), which isdesirable.

Other types of polymers yield films with large signals but long responsetimes (e.g. many seconds). In respiratory oxygen measurements, veryshort response times (ca. 100 milliseconds) are desirable. Consequently,films made from polymers with the characteristics just discussed areless than satisfactory or even completely unusable.

Nevertheless, there are polymers from which membranes (or matrices) thatare suitable for sensors as disclosed herein can be made. These includeporous polyvinyl chloride, polypropylene, polycarbonate, polyester,polystyrene, and polymethylacrylate polymers and acrylic copolymers.Those materials resemble thin sections of porous sponge with a highvolume fraction of air space. They are ideal for introducing a solutionwhich can be absorbed into the polymer, yielding an altered membranewith the luminescable composition molecularly dispersed in the polymericmatrix.

Representative of the polymers identified above which are usable forsensors embodying the principles of the present invention aremicroporous polycarbonates marketed by Gelman-Sciences, Whitman, andOsmonic/Poretics. Currently preferred are the track-etched, microporousfiltration membranes of Osmonic/Poretics. Track-etched polymers have theadvantage in the context of the present invention that the particles ofluminescable composition are readily captured on the surface of thesensor matrix.

Irrespective of the polymer which is selected, it is preferred that thesensor film or membrane have a thickness of 5 to 20 μm and a pore sizeranging 0.1 to 10 μm as the diffusion constant for oxygen in films ofthose parameters is large enough to provide a response time ofsufficiently short duration.

The Osmonics/Poretics track-etched polycarbonate with a 0.4 μm pore sizeand a thickness of 10 μm is the preferred membrane material. Thismaterial has many advantages. Its thin porous structure facilitatesincorporation of the luminescable composition into the polymeric matrix,such that all of the composition in the matrix is only a short distancefrom the gases being monitored. This allows oxygen in those gases torapidly diffuse into the matrix and into contact with the luminescablecomposition. Fast diffusion translates into a fast response to theoxygen in the gases being monitored.

The uniform polymeric structure of the Poretics material gives rise toeasily manufactured matrices with the same, reproduciblecharacteristics. This polycarbonate film has excellent signal response;i.e., change of signal with change in oxygen. Also, these films seem toshow a higher degree of photstability; i.e., less change or photo-agingover a given time of luminescence.

As indicated above, a number of techniques can be employed in accordancewith the principles of the present invention to disperse theluminescable composition in the polymeric matrix.

The composition may be dissolved in an appropriate solvent which iscapable of swelling the polymeric material, thereby allowing theluminescable composition to be readily introduced into the matrix. Thesolvent interacts strongly with the polymer material, but theinteraction is not so great as to cause the polymer to dissolve in thesolvent. Since this solvent has the luminescable composition dissolvedin it, the swelling of the polymer by the solvent carries thecomposition into the polymer matrix. The impregnated matrix is thendried to evaporate the solvent. When the solvent evaporates, theluminescable composition is left behind, incorporated in and molecularlydistributed within the polymer matrix.

Others have attempted to disperse a luminescable composition in apolymeric matrix by dissolving the composition in a mixture containingthe monomer or polymer precursors and then initiating a polymerizationreaction. The methods disclosed herein and utilizing swelling of thepolymer to introduce the luminescable composition into the sensor matrixhave the advantage of being simpler and more reproducible and ofallowing the use of virtually any polymer which does not appreciablydissolve when the solvent is applied.

A variety of solvents are suitable. These include hexane, petroleumethane, toluene, tetrahydrofuran, methylene chloride, trichloroethylene,xylene, dioxane, isopropyl alcohol, butanol and mixtures including thosesolvents with the particular solvent depending upon the polymer andluminescable composition that are employed.

The solution of luminescable composition in swelling solvent may bepainted onto the polymeric membrane. The polymeric matrix can, in analternate approach in accord with the principles of the invention, besoaked in the solution of luminescable composition and swelling solvent.

In the approaches just described, the solvent is removed by drying themembrane in air or by gas, leaving the luminescable compositiondispersed and trapped in the polymeric matrix.

In yet another sensor fabrication technique embodying the principles ofthe present invention, the swelling solventiluminescable compositionsolution is sandwiched between two thin polymeric membranes such thatthe two membranes become solvent bonded together. This can beaccomplished either by using an attacking solvent in the solution or byfurther application of an attacking solvent or solvent mixture to thetop membrane.

In this process of fabricating an oxygen sensitive sensor, theluminescable composition is effectively introduced into the center ofthe resulting polymer film; and, when the two polymer layers are fusedtogether, the sandwiched luminescable composition is incorporated in thestructure of the polymer. The attacking solvents must dissolve both thecomposition and the polymer to some extent so the composition can becarried into the polymer matrix. -Since the luminescable compositionpenetrates from the center of the resulting film, this is a way toincorporate a greater quantity of luminescable composition into a filmto have a higher concentration of luminescable composition in the centerof the thin dimension of the film, where it is less exposed and moreprotected.

Due to the thin, porous structure of the starting polymers, sensor filmsembodying the principle of the present invention have the advantage thatthe molecules of the luminescable composition are embedded withinmicrons of the gas-polymer interface and have fast response times. Thepores serve as channels for introducing the composition into the porouspolymeric matrix, allowing a three-dimensional incorporation of thediffusing composition. Where the solvents do not strongly attack thepolymer structure, the pores survive; and gas diffusion into the polymermembrane is enhanced due to the short diffusion distances. Since thestarting material is a thin polymer with a high degree of manufactureduniformity, batch processing of sensors of uniform composition andcharacteristics is facilitated.

Irrespective of the process that is selected, the characteristics of thesensor can be modified, if advantageous, by overcoating the impregnatedpolymeric matrix with an additional polymeric film. This can beaccomplished by painting a dilute solution of the polymer in a volatilesolvent on the matrix or by solvent bonding an additional thin polymericmembrane on the surface of the film.

Overcoating is used to refine the characteristics of the film. Theprimary characteristics of the luminescence result from the particularluminescable composition and the particular polymer in which it isdissolved. Characteristics of the overall sensor that may be modified byovercoating the matrix polymer include light absorption and transmissionproperties, gas permeability, and interaction with solvents or otherchemical substances. For example, if a sensor film consisting of a dyein a given polymer has nearly ideal characteristics, but it is desiredto decrease the level of diffusion of oxygen into the film, this may beaccomplished by overcoating the film with a thin layer of a polymerwhich is less oxygen permeable than the matrix material. The desirableproperties of the original film are retained but have a less sharpresponse to oxygen, more signal, and less quenching. The overall effectis to provide the matrix film with characteristics that can not beobtained by simply dissolving the luminescable composition in existingmaterial.

The following example presents one representative method for making anoxygen sensor embodying the principles of the present invention.

EXAMPLE

A 10 μm thick Poretics® track-etched polycarbonate with a 0.4 micronpore size supplied by Osmonics/Poretics, Livermore, Calif.) is used asthe polymeric matrix of a sensor. A mixed solvent is prepared by mixing3 ml of methylene chloride (Mallinckrodt, UltimAR®) with 7 ml of toluene(Mallinckrodt, AR®). To 10 ml of this mixed solvent is added 15 mg ofplatinum meso-tetra (pentafluorophenyl) porphine (Pt TFPP, PorphyrinProducts, Logan Utah ). Slight stirring of the mixture gives completedissolution of the porphyrin, resulting in a red-orange dye solution.

One-inch disks or one-inch squares of the polycarbonate film are placedseparately in the bottom of a small glass beaker or on top of a glassplate, and the dye solution is added dropwise to the film pieces untilthey are saturated with solution. Over several minutes, a gentlebuckling and swelling of the film is evident, after which several dropsof dye solution are added to each film segment.

As the solvent begins to evaporate, the film pieces are transferred toother glass slides with forceps clipped to a small wooden stirring stickor a wire and hung over the top of a small, empty beaker. Excessremaining solution is then washed from the film surface by transferringisopropyl alcohol from a pipet to the top of the hanging film andallowing the alcohol to drip off the bottom surface. Afterwards, eachfilm piece is dried and cut to size for mounting in airway adapters asshown in FIG. 6 of the drawings.

Turning now to FIGS. 7-10 of the drawings, an important feature of thepresent invention is a novel process for installing sensor 47 in anaperture 62 which is formed in the wall 68 of airway adapter centralsection 48. A sensor blank 82 is placed between thermal capacitor 78 andairway adapter wall 68 (Step 1, FIG. 7) and then lowered (Step 2, FIG.8) until the blank 82 rests on side wall 68 in overlying relationship toaperture 62 (Step 3, FIG. 9). Then, thermal capacitor 78 is displaced inthe direction indicated arrow 84, pushing the sensor blank 82 toward thebore 62 of airway adapter central section 48 as thermal capacitor 78moves into aperture 62 (Step 4, FIG. 10). Friction between the domedside 86 of the thermal capacitor and sensor blank 82 and between theblank and the periphery 88 of airway adapter aperture 62 causes blank 82to be stretched over the domed surface 86 of the thermal capacitor 72 asthat airway -adapter component moves to the installed position of FIG.10. This tightens the blank against thermal capacitor surface 86 andprovides firm, intimate contact between the sensor and the thermalcapacitor. This is important because the energy outputted by sensor 47when it is excited into luminescence is very temperature dependent. Withintimate contact between the sensor and thermal capacitor, temperaturevariations of sensor 47 during the operation of system 30 can be reducedto an acceptable minimum, if not entirely eliminated, by controlling thetemperature of the thermal capacitor with an important and novel way ofaccomplishing this objective being discussed hereinafter.

A circumferential lip 92 is provided at the inner end of aperture 62 inairway adapter central section 48. This lip stops the assembly 72 ofthermal capacitor 78 and sensor 47 at the proper location relative tothe boundary of the bore 62 through airway adapter element 48. A secondcircumferential lip 100 at a location intermediate the inner and outerends of aperture 62 holds assembly 72 in place in the designatedposition and keeps the assembly from popping out of the airway adapterwall 68.

Referring now primarily to FIGS. 1, 4, and 11-13 of the drawings,transducer 34 is employed to excite sensor 47 into luminescence and toconvert the light emitted by the excited sensor to an electrical signalindicative of the oxygen concentration in the gases flowing throughairway adapter passage 60. The transducer includes a casing 106 composedof casing components 108 and 110. Housed in casing 106 are an opticalsubassembly 112, a sensor heater system 114, and a printed circuit board(PCB) 116.

More specifically, and as is shown in FIGS. 4 and 13, transducer casingcomponents 108 and 110 have cavities 118 and 120 defined by the side andend walls 122 and 124 of casing component 108 and by the side and endwalls 126 and 128 of casing component 110. These cavities cooperate todefine an enclosed compartment (or well) 130 in which thejust-enumerated components or sub-assemblies of transducer 34 arehoused. A lip 132 on the side wall 126 of casing component 110 fits intoa complementary recess 134 of side wall 122 to fix the two casingcomponents 108 and 110 together and to provide a tongue and groove sealwhich keeps water and other foreign material from penetrating intocasing compartment 130.

Optical assembly 112 is placed in casing component 110 and fastened inplace by screws 136-1 and 136-2 which extend through apertures 138-1 and138-2 of the platform 140 of the optical assembly into blind tappedapertures (not shown) in component 110. Casing component 108 is thenplaced over the optical subassembly and fastened in place with threescrews 142-1, 142-2, and 142-3 (FIG. 4). Screw 142-1 extends throughboss of casing component 108 (FIG. 4) and aperture 145 in platform 140(FIG. 11) into blind, tapped aperture 146 in boss 147 of casingcomponent 110. Screws 142-2 and 142-3 extend through apertured bosses incasing component 108 directly into blind, tapped apertures in bosses ofcasing component 110 (the casing 108 boss and the complementary boss incomponent 110 for screw 142-3 are shown in FIG. 4 and identified byreference characters 148 and 150.)

Referring now primarily FIGS. 4 and 11-13, the optical assembly 112 oftransducer 34 includes the above-eluded-to platform 140, a light source154, data and reference detectors 156 and 158, signal processingcircuitry 162 (See FIGS. 17, 18, and 20) and a beam splitter 162. Datadetector 156 and reference detector 158 are conventional PIN photodiodessupplied by Centronic, Newbury Park, Calif., and beam splitter 162 maybe as simple as a piece of clear glass or plastic.

Light source 154 is mounted in a socket 167 formed in optical platform140. Bright green and blue LED's are essentially ideal light sources.These LED's have high intensity in the needed luminescable compositionabsorption region with little non-useful output at other wavelengths,especially near ultraviolet. This minimizes stray interfering light andphotodegradation of the sensor.

Other advantages of these LED's are light weight, compactness, low powerconsumption, low voltage requirements, low heat production, reliability,ruggedness, relatively low cost, and stability. Also they can beswitched on and off very quickly, reliably, and reproducibly. Arepresentative light source is a bright green LED supplied by NichiaChemical Industries, Ltd. and modulated (i.e., turned on and off) at afrequency of 4 kHz.

LED 154 is oriented with its axis of propagation 169 (see FIG. 13) at a45° angle to sensor 47. The light emitted from source 154 is refocusedinto a beam by a lens 171 also installed in platform 140. The beam ispropagated along optical path 172 to excite the luminescable compositionof sensor 47 into luminescence. Oxygen in gases moving through the flowpassage 60 in airway adapter 36 quenches the luminescence exhibited bysensor 47 in a way which reflects the concentration of oxygen in thosegases. The excited sensor composition emits light in the red part of theelectromagnetic spectrum.

The emitted energy is propagated-along optical path 173 through beamsplitter 162 to data detector 156. That optical assembly component ismounted in a recess 174 located in an inclined element 175 of opticalplatform 140 in line with an opening 176 through that platform element.

Light of a wavelength which can be processed into a signal indicative ofthe concentration of oxygen in the gases flowing through airway adapterpassage 60 is transmitted by beam splitter 162 through aperture 176 todata detector 156 as indicated by arrow 177. Light is also reflected bybeam splitter 166 through filter 178 to reference detector 158 asindicated by arrow 179.

It is important, from the viewpoint of accuracy, that onlyelectromagnetic energy containing oxygen concentration data reach datadetector 156. This is accomplished in transducer 34 with filters and byestablishing a particular geometric relationship between light source154 and data detector 156. More specifically, a filter 180, green for agreen LED and blue for a blue LED, is mounted on optical platform 152between LED 154 and lens 171, and a red filter 182 is mounted to thatplatform between beam splitter 162 and data detector 156. Red filter 182screens from data detector 156 all but the red light indicative of theoxygen concentration in the gases flowing through airway adapter passage60.

A very small fraction of the light emitted by LED 154 falls in the redpart of the visible spectrum. LED filter 180 (green or blue) keeps thisred light from reaching sensor 47, thereby promoting the accuracy of theoxygen concentration as seen by the data detector.

A fraction of the light emitted by LED 154 is not absorbed by sensor 47but is reflected from the sensor along path 177, for example. A smallpart of that light is reflected by beam splitter 162 onto referencedetector filter 181, passing through that filter to reference detector158. Filter 181 will typically be green or blue, depending on the colorof LED 154. The filter consequently screens out any red light indicativeof oxygen concentration reaching the reference detector. Consequently,the light reaching the reference detector contains only data which isnot indicative of oxygen concentration and can accordingly be used tocorrect changes due to the LED or optical path as one example.

Referring now most specifically to the pictorial representation ofoptical assembly 112 in FIG. 14, a part of the light emitted by LED 154is not absorbed by the luminescable composition in sensor 47 but isinstead reflected from this sensor as shown by the dotted linescollectively identified by reference character 186. This specularreflection is kept from data detector 156 and interfering with theaccuracy of the oxygen-indicative signal produced by the data detectorby making the angle between LED 154 and data detector 156 such thatreflected rays of light do not reach the data detector. Instead, onlythe light emitted by the luminescing composition, shown in solid lines188 in FIG. 14, and carrying oxygen concentration information reachesdata detector 156.

Referring now to FIGS. 5, 6, 13, 15, and 16, heretofore unaddressed isthe necessity of maintaining the sensor of a luminescense quenchingoxygen monitoring system at a constant temperature. This is necessarybecause as mentioned above, the emission of light from the luminescablecomposition in sensor 47 is very temperature sensitive, because changingflow rates and the temperature of the gases being monitoredsignificantly effect the temperature of the sensor, and because thepolymeric matrix of the sensor is by itself not capable of rapidlyresponding to temperature changes in the gases being monitored. Inexemplary oxygen concentration monitoring apparatus 30, this problem issolved by: the use of thermal capacitor 78 in conjunction with theheating system components shown in FIGS. 13, 15, and 16, the aggressiveheater control shown in FIG. 19 and identified by reference character190, and with the above-described novel technique for so installingsensor 47 in airway adapter casing 50 that the sensor is stretchedtightly over, and remains in an intimate, heat transfer promotingrelationship with, the thermal capacitor 78.

Sensor heating system 114 includes, in addition to thermal capacitor 78,a thermally conductive base 192, a thick film resistance heater 194, anda temperature sensor 196. Heating system base 192 is installed in anaperture formed by complementary moon-shaped recesses 200 and 202 (seeFIG. 4) in side walls 122 and 126 of transducer casing components 108and 110. A lip 203 surrounding aperture 200/202 is trapped in a recess204 which extends around the periphery of the installed heater systembase component 192 to retain that heating system component in place.

Referring now most specifically to FIGS. 13, 15 and 16, airway adapter36 is removably assembled to transducer 34 by displacing the airwayadapter in the direction indicated by arrow 205 in FIGS. 15 and 16 withthe airway adapter center section 48 sliding into a complementary recess206 defined by recess elements 207 and 208 in the side walls 122 and 126of transducer casing components 108 and 110 (see FIG.4) until a flange208 a on the airway adapter central section 48 contacts transducercasing 106. As airway adapter 36 slides into transducer 34, the flatback side 209 of airway adapter thermal capacitor 78 comes into contactwith the also flat, front side 210 of the heating system base component192 in transducer 34. This provides intimate physical contact betweenthe heater base and the thermal capacitor, insuring efficient, uniformtransfer of heat from the heater base to the thermal capacitor.

This intimate contact is promoted and maintained by a compressibleO-ring 211 installed in heater base circumferential groove 204 betweenside wall elements 212 and 214 of transducer casing component side walls122 and 126. The O-ring 211 lies between: (a) side wall elements 212 and214 and, (b) a groove bounding lip 216 at the airway adapter facing,flat side 210 of the heater base and is compressed as airway adapter 36slides into transducer 34 and as is suggested by arrow 217 in FIG. 16.The tendency of O-ring 211 to return from the compressed state shown inFIG. 16 to the unstressed state shown in FIG. 15 promotes the wantedintimate contact between the heater base and thermal capacitor 78 bybiasing the heater base toward the thermal capacitator.

The dimensioning of heater base peripheral recess 204 relative to thethickness of transducer casing side wall elements 212 and 214 providesfor relative movement between heater base 192 and the transducer casing106 in the arrow 217 direction. That movement compensates for anystructural misalignments or variations in dimension between airwayadapter 36 and transducer 34.

Turning now to FIGS. 2, 3, and 5, it is critical to the performance ofsystem 30 that airway adapter 36 be oriented in the correct relationshipto transducer 34 (shown in full lines in FIG. 3) rather than in theopposite relationship shown in phantom lines in the same figure.Incorrect assembly is precluded by stops 218 and 220 on transducercasing end wall segment 128 and complementary stops 222 and 224 onairway adapter end segment 52 (See FIG. 15). Any attempt to installairway adapter 36 in transducer 34 in the wrong, phantom lineorientation results in the airway adapter stops 222 and 224 engagingtransducer casing stops 218 and 220, preventing the airway adapter frombeing coupled to the transducer.

Referring now to FIGS. 4 and 13, it was pointed out above thattransducer 34 includes a PCB 116 on which various circuits andelectrical components of the transducer operating systems are mounted.PCB 116 is supported in PCB guides 228 and 230 located at opposite sidesof transducer casing 106 (the top and bottom sides of the transducerwith that system component oriented as shown in FIG. 13). Lower guide228, as shown in FIG. 4, is made up of spaced apart lugs 232 and 234 incasing component 108 and lugs 236 and 238 in casing component 110. Thedistance between the lugs 232-234 and 236-238 is slightly greater thanthe width of PCB 116 so that the PCB can be readily fitted into the PCBguides.

The upper guide PCB 230 essentially duplicates the lower guide 228.Those PCB guide segments in transducer casing component 110 are shown inFIG. 13 and identified by reference characters 239 and 240. Thesesegments are duplicated in mirror image relationship in casing component108.

Leads collectively identified by reference characters 241 and 242 extendthrough aperture 243 in transducer casing component 110 and areincorporated in the external cable 40 which connects the circuitry onPCB 116 to the hand held control and display unit 38.

Referring still to the drawing, reference character 244 in FIG. 17identifies the operating system of transducer 34. Also shown in FIG. 17are LED 154, LED filter 180, thermal capacitor 78, heating system base192, temperature sensor 196, photodiode data detector 156, and datadetector filter 182. The display 46 and data processing computer 246 ofhand held unit 38 are also shown in block diagram in that figure.

Operating system 244 includes a conventional driver 266 for LED 154 andheater control 190 which is preferably of the PID(proportional-integral-differential) type. The heater control acceptstemperature data from sensor 196 and, based on the sensed temperature,controls the flow of current to the thick film resistive heating element194 of the sensor heating system 114 in transducer 34.

Operating system 244 also includes an amplifier 270 for the oxygenconcentration indicative signal outputted by photodiode data detector156 and a lock-in amplifier 272. The signal from the lock-in amplifieris further processed in the computer 246 of unit 38 and converted into areading for display 46. A clock 274 controls the operation of LED driver266 and lock-in amplifier 272.

The lock-in amplifier circuit shown in FIG. 18 and identified byreference character 247 possesses especially attractive and simpleprocessing of the phosphorescence decay signals from oxygen sensor 47.In the FIG. 18 circuit, a square or sine wave generated at a selectedfixed frequency by clock generator 274 (FIG. 17) provides an inputsignal which is amplified by amplifier 275. This amplified frequency isused to modulate the light output of LED 154 and serves as a referencefor lock-in amplifier 272. The lock-in amplifier only detects signals atthe same frequency as this reference, thereby rejecting all d.c. signalsand nearly all signals at any other frequency. This enables detection ofweak signals having a strength which is orders of magnitude well belowthe level of all electronic noise in operating system 244.

The rise and decay times of the luminescence generated by the excitedsensor 47 cause the signal generated by data detector 156 to have aphase lag with respect to the wave form of original LED driver 266.Measurement of this phase lag is the equivalent of measuring theluminescence decay time, which is the oxygen-dependent parameter ofinterest. In the FIG. 18 dual-phase lock-in detection circuitry, asecond reference phase is generated at the same frequency as the first,but with a phase lag of exactly 90°. In each of the two synchronousdemodulators 277 and 278 in the FIG. 18 circuit 276, the data signal ismultiplied by one of these two phase references. This produces tworesultant signals, which are the in-phase and quadrature components ofthe original signal from data detector 156. For a static signal, thesetwo outputs are d.c. voltages. This is another advantage of lock-inamplification in that the signal processing circuitry needs to handleonly the analog-to-digital conversion of two d.c., slowly varyingvoltages. The amplitude and phase of the signal is gotten by simplecalculations from these two voltages as follows:

A=(V_(I) ²+V_(Q) ²)^(½)

θ=tan⁻¹ (V_(Q)/V_(I))

where:

A=amplitude

θ=phase angle

V_(Q)=quadrature voltage

V_(I)=in-phase voltage

This detection scheme with its simplicity of operation is a significantfeature of the present invention. Direct measurement of the decay timesin the heretofore proposed microsecond range requires an electronicsampling system running at megahertz frequencies. In systems asdisclosed herein, in contrast, optimization of phase detection occurs atmuch lower frequencies (ca. 5-25 kHz). That this is true is importantbecause it greatly simplifies the electronic circuitry.

As discussed above, clock 274 provides a square or sine wave signal.This signal is used to produce a modulated light output from LED 154,which follows the driver 266 to which the clock is coupled. Themodulated light from the LED excites sensor 47 into luminescence. Thisluminescence (or phosphorescence) has a time decay which is dependentupon the oxygen concentration in the medium bathing sensor 47. The lightemitted by the luminescing sensor is detected by silicon PIN photodiode156 where it is converted into a current, then amplified and sent to theinputs of the dual-channel LIA 272. This current signal looks like thereference or driver wave form with a phase shift or delay proportionalto the phosphorescence decay time. Two lock-in amplifier outputs, V(in-phase) and V (quadrature) are sampled by an analog to digitalconverter (not shown) in the computer 246 of hand held unit 38. Theamplitude and phase of the signal are then calculated by computer 246from the two voltages.

In a calibration mode, the decay times or phase angles are measured as afunction of standard calibration gases of known oxygen concentration,and the values are entered into a calibration file in the computermemory. For oxygen measurement, the lock-in voltages and resultant phaseangles are collected and averaged in computer 246 and converted tooxygen levels using the calibration file and an interpolation or fittingroutine. The calculated oxygen level is then displayed on the display 46of hand held unit 38.

The control system 190 for sensor heating system 114 is shown in moredetail, albeit still in block diagram form, in FIG. 19. Sensor heatersystem 190 uses a proportional-integral-differential (PID) heatercontroller 279 for active temperature stabilization of thermal capacitor78 and oxygen sensor 47. To this end, the temperature of the heat sink(or heater base) 192 in transducer 34 as provided by the temperaturesensor 196 mounted thereon is converted to a temperature indicativevoltage input to the PID circuit. This voltage, amplified with amplifier288, is compared within the circuit by a comparator 280 with apot-settable voltage representing the temperature setpoint.Proportional, integral and differential comparisons of the sensed andsetpoint temperature signals over time are developed by PID circuit 279as indicated by the boxes labeled 282, 284, and 286. More specifically,the temperature voltage is amplified (amplifier 288) and the referencetemperature voltage is subtracted from it. The resultant temperatureerror voltage is amplified (amplifier 290) and split into three paths:Proportional (P), Integral (I), and Differential (D).

The Proportional path represents the temperature error magnitude, theIntegral path represents the integral of the temperature error overtime, and the Differential path represents the rate of change of thetemperature error. The three paths are summed by a summing junction 291at the input of amplifier 292. The amplifier output drives theresistance heater 194 which is also mounted on heater base 192.

Thermal feedback is provided by heater component 192 (FIG. 13) which ischosen for good thermal conductivity. Tuning the circuit for the thermalcharacteristics of heater base 192 and heater 194 results in anoperating voltage which, amplified by amplifier 292, is aggressivelyapplied to the resistive heater 194 in contact with heat sink 192whenever a decrease in temperature is detected. Likewise, the PID heatercontrol 190 quickly reduces the rate of heating as the heat sinktemperature approaches the set temperature and cuts off heating when thetwo temperatures match. Since the thermal sink (heater base) 192 is heldwell above ambient temperature, cessation of heating results in theonset of rapid cooling. This is immediately detected by PID circuit 279by virtue of the thermal feedback from heater 194 to temperature sensor196 as indicated by line 294 in FIG. 19. The result is the applicationof frequent pulses of heat to heater base 192, stabilizing it and sensor47 within a narrow range (one or two tenths of a Celsius degree) nearthe setpoint.

Heat transfer from heating system component 192 by conduction is alsoinstrumental in keeping moisture from condensing on airway adapterwindow 74. This is significant because moisture condensed on window 74can adversely affect the accuracy of the oxygen concentrationmeasurement made by system 30 to a significant extent.

The three-fold way in which PID circuit 279 “decides” to respond totemperature change allows the heater control system 190 to respondrapidly to conditions such as those appurtenant to large gas flows,resulting in only minimal variation in the heater base and sensor 47temperature. Even with the sizeable temperature dependence of thesensor, the temperature control just described responds to the sensortemperature changes so fast as to suit it for breath-by-breath analysisapplications of the present invention.

In more detail, the PID heater control circuit 279 works by having atemperature setpoint, Ts, represented by a corresponding voltage. Themeasured temperature T is represented by a voltage developed bytemperature measurement element 196 which may be a thermocouple orthermistor, and T is compared to Ts by comparator 280 as describedabove. PID circuit 279 applies a heating voltage proportional to thetemperature difference as follows:

P=GpX (Ts−T)

where:

P is the heating voltage,

Ts−T is the difference between the detected temperature and thetemperature setpoint, and

Gp is the proportional gain of the circuit.

This proportional heating approach gives more precise temperaturecontrol than simple on-off heating, but is still not generallysufficient for tight temperature control at temperatures near thesetpoint temperature. This is because, as T approaches Ts, theproportional difference is small. At small gains, very little heat isdelivered to heater base 192, and the time to heat the base to Ts islong or even infinite. Increasing Gp, the gain, to decrease the heatingtime has the effect of causing heating overshoot. This in turn causesinstability of the sensor temperature as the system must turn off andcool by natural heat loss. The overshoot at high gain is remedied inpart by adding the differential temperature control circuit. Thisoffsets the tendency of the heater control system 190 to overshoot thesetpoint temperature by damping the heating when a high rate of changeof Ts−T is detected. Proportional heating always tends to settle belowthe setpoint, however. To correct this, the signal is designed tobootstrap the system to a temperature close to the setpoint temperature.Specifically, the integral circuit integrates the difference between thesetpoint and measured temperatures and applies heat to force theintegral near zero. This effectively counteracts the residualtemperature difference from the proportional circuit, resulting in thesensor setpoint temperature T being maintained very near, if not at, thesetpoint temperature Ts.

Resolution of small differences in oxygen partial pressures places highdemands on instrument performance in a luminescence-quenching system.This is particularly true for higher oxygen concentrations. Theluminescence quenching-based process produces a signal over a highdynamic range; in going from 0 to 100% oxygen, the signal amplitudehalves several times over. Likewise the decay time decreases by a decadeor more. Phase-sensitive or lock-in detection which is preferred anddescribed above is run at a detection frequency that is optimized forthe transition time being detected, making it possible to measure smalldifferences in the transition time—in systems the employing theprinciples of the present invention the luminescence decay time. Phaseoptimization occurs at a combination of frequency, f, and decay time, t,where the detected phase angle is 45°. When the decay time changesten-fold, the optimal detection frequency also changes ten-fold. Overany such measurement range, choice of one particular detection frequencymay therefore seriously degrade the resolution of the oxygenconcentration signal.

There is additional negative impact from the large change in amplitudeof the signal. Generally, it is desirable to operate stages of thelock-in amplifier circuit 276 at high gain so that any noise enteringthe system will have minimal effect on the signal-to-noise ratio.Optimization of the widely-varying signal presents a problem. In orderto have a sizeable magnitude of the much-diminished signal at highoxygen concentrations, the low-oxygen signal is often “offscale” of adevice such as an analog-to-digital converter. Conversely, with alow-oxygen signal employed full scale, the high-oxygen signal is smallenough to suffer degraded resolution.

A signal processing system of the character shown in FIG. 21 andidentified by reference character 298 solves this problem by providing adual measurement scale to enhance the accuracy of measurement of oxygenconcentration indicative signals of widely varying amplitude and decaytime.

As indicated by signal processing branches 300 and 302 in FIG. 20, twosignal modulation frequencies are chosen, one such that the phase anglesof signals for a low oxygen range will be close as possible to 45°. Thesecond modulation frequency is chosen to meet the same criterion for thephase angles of signals indicative of high oxygen concentrations. Thisprovides at both high and low concentrations of oxygen in the gasesbeing monitored a signal which has a high degree of resolution yet canbe handled by a conventional analog-to-digital converter.

As suggested by boxes 304 and 306 in FIG. 21, low oxygen concentrationscan, in applications of the present invention involving breath-by-breathmonitoring, typically be defined as those with less than 30 percentoxygen and high oxygen concentration ranges as those having more than 30percent oxygen.

The signal processing circuitry 298 of oxygen monitoring system 30 isswitched between the high and low ranges (decision box 308) manually orautomatically in response to the detection of a parameter such as decaytime.

With the signal processing circuitry switched to the LOW range setting(box 316), the phase of the data signal generated by detector 154 ismeasured as indicated by box 318. Next, the oxygen concentrationcorresponding to the measured phase angle is looked up as from adigitally stored calibration curve (box 320); and the concentration isshown (box 322) as on the display 46 of unit 38.

Equivalent steps are employed if the oxygen concentration being measuredis in the HIGH range and the signal processing circuitry is manually orautomatically switched to the high range frequency modulation (referencecharacter 309) setting. The phase angle of the data detector generated,oxygen concentration indicative signal is measured (box 310); and thecorresponding oxygen concentration is looked up (box 312) and shown ondisplay 46 of unit 38 (box 314).

A similar scheme can be employed for an amplitude scale. An automaticgain control (AGC) circuit may be used to keep the signal level at aconstant amplitude, or a dual gain setting can be established inconjunction with the frequency scale.

It will be remembered that the exemplary oxygen concentration monitoringsystem 30 disclosed herein employs both a data detector 156 and areference detector 158. Using a reference signal to periodicallycalibrate data detector 156 increases the accuracy of the oxygen valuedisplayed by unit 38. As shown in FIG. 20, the reference detector 158 isswitched into the signal processing circuit with the computer 246switching from a data measuring mode to a calibration mode whilereference detector 158 is active. The reference signal is processedthrough the lock-in amplifier circuit 276 with the processed signalbeing transmitted to computer 246 to switch the computer to its “timeout for reference check mode”. The reference detector signal is alsosent directly to computer 246 to provide a signal which the computer canemploy to apply an appropriate correction to the signal generated bydata detector 156.

The advantages of basing an oxygen concentration indicative signal on adata signal corrected by a reference signal can alternatively berealized by using data and reference light sources and a singledetector. A transducer employing this scheme is illustrated in FIGS.22-24 and identified by reference character 330.

For the most part, transducer 330 is like the transducer 34 describedabove. Components of the two transducers which are alike willaccordingly be identified by the same reference characters.

Transducer 330 differs from transducer 34 in one significant respect inthat the platform 332 of the transducer 330 optical assembly 334 isconfigured to support a second reference light source 336 such as anorange red, or ultraviolet LED. The two LED's 154 and 336 are mounted inside-by-side relationship in platform 332.

Transducer 330 also differs significantly from transducer 34 in that itemploys only a single detector 342 which supported from platform 332 inthe optical path 346 between sensor 47 and the detector. Detector 342 isa data detector and generates a signal indicative of the concentrationof oxygen in the gases being monitored.

Light from reference LED 336 does not excite the luminescablecomposition in sensor 47 but is reflected from the sensor, passingthrough filter 344 to photodetector 342 and producing an electricalsignal related to the intensity of the light emitted from LED 336. Thesignal so obtained may be used as a reference phase correction fordrifts in the electronic system. The second LED 336 may be switched onto provide a reference or calibration point from time to time as desiredor may be switched on for regular, short intervals to provide a nearlycontinuous automatic reference.

The exemplary oxygen concentration monitoring system 30 shown in FIG. 1of the drawing and discussed above is of the mainstream, online type.However, the principles of the present invention can equally well beemployed in sidestream sampling systems. One representative system ofthis character is shown in FIGS. 25-27 and identified by referencecharacter 350.

System 350 includes a nasal canula 352, an oxygen concentration monitor354 embodying the principles of the present invention, absolute anddifferential pressure transducers 356 and 358, a barometric pressureport 360, a vacuum pump 362, and a damping chamber 364.

Nasal canula 352 (FIG. 26) is conventional. It includes tubing 366 whichfits over the head of a patient 368. An insert 370 in the tubing hasnipples (one shown and identified by reference character 374) that fitinto the patient's nostrils. The nasal canula is connected as by tubularfitting 376 to a flexible Nafine drying tube 378. The drying tuberemoves moisture from gases exhaled by patient 368, thereby eliminatingerrors which that moisture might cause. At the far end of the tube isthe female component 380 of a conventional Leur fitting.

Referring now specifically to FIGS. 25 and 27, the oxygen concentrationmonitoring unit 354 of sidestream sampling system 350 includes an oxygenconcentration signal generating transducer 386 and a removable samplingcell 388. The sampling cell 388 has a casing 390 which terminates in amale Leur fitting 392 which complements the female component 380 of thatfitting shown in FIG. 26. The two Leur fitting components are pluggedtogether to establish fluid communication from the patient 368 beingmonitored through nasal canula 352 and drying tube 378 to a flow passage394 extending from end-to-end through sampling cell 388. A filter 396 isinstalled in flow passage 394 to remove any remaining moisture and otherforeign material from the patient's exhaled gases before those gases aremonitored for oxygen content.

Mounted in the casing 390 of transducer 386 is a luminescable oxygensensor 398 supported by a thermal capacitor 400. Opposite thesensor/thermal capacitor assembly is a transparent window 402. Thosecomponents are akin to the correspondingly named components of thesystem shown in FIG. 1 and will accordingly not be described furtherherein.

Referring still primarily to FIGS. 25 and 27, the casing 390 oftransducer 386 has aligned apertures 404 and 406 with sample cell 388extending through those apertures. Housed in a compartment or cavity 408in transducer casing 390 are a LED light source 410, a data detector 412for light emitted from the sensor 398 of sample cell 388, and a sensorheating system 414 which cooperates with sample cell thermal capacitor400 to keep oxygen concentration sensor 398 at a constant temperature.

Heating system 414 includes a conductive heating element 416, aresistance heater 418, and a temperature sensor 420. The just-namedcomponents are like those employed in the oxygen monitoring system 30discussed above, and they are mounted in transducer casing 390 in muchthe same manner as the components of the corresponding online transducer34. Consequently, and in the interest of avoiding unnecessaryrepetition, the just-identified internal components of transducer 386will not be further described herein.

As is the case with an online system such as shown in FIG. 1, theelectrical signals generated by the data detector 412 of transducer 386are transmitted to a control/signal processing unit, in this caseidentified by reference character 422 and shown in FIGS. 25 and 27.Functions and the capabilities of unit 422 are also identified in FIG.27.

Referring again to FIG. 25, an operator utilizing sidestream samplingsystem 350 for the first time operates a switch (not shown) to applyelectrical power to the system. This results in a three-position valve424 being moved to the NO position to equilibrate system 350 withbarometric atmospheric pressure through lines 426, 428, and 430 andbarometric pressure port 360 and to provide a barometric pressure valueand a flow pressure differential. The just-identified lines provide aflow path 434 between barometric pressure port 360 and: (1) absolutepressure transducer 356, (2) differential pressure transducer 358, and(3) a sidestream sampling line 437. The sampling line continues the flowpath from the sampling cell 388 of oxygen concentration monitoring unit354 to: (a) vacuum pump 362, and (b) damping chamber 364. The barometricand flow pressure differential values are stored in processing signalcontrol unit 422 and employed as reference during the operation ofsystem 350.

After the reference pressure is stored, valve 424 is moved to the NCposition. This applies atmospheric pressure to the absolute pressuretransducer 356, which transmits a signal indicative of the barometricpressure to unit 354. As just suggested, this pressure is utilized as areference in the operation of sidestream sampling system 350.

During the operation of system 350, valve 424 is maintained in the COMposition. This connects absolute pressure transducer 356 anddifferential pressure transducer 358 to sidestream sampling line 437through lines 426, 428, 430, and 432. Differential pressure transducer358 is also connected to the sidestream sampling line 437 by line 440and orifice 442. This applies two different pressures acrossdifferential pressure transducer 358, resulting in the differentialpressure transducer having an output which represents the rate of flowof the gases being monitored along flow path 434.

With pressure transducers 356 and 358 connected to sidestream samplingline 437, vacuum pump 362 is operated. The motor (not shown) of vacuumpump 362 is voltage controlled by a loop that includes differentialtransducer 358 such that a uniform flow of gas is maintained through thesampling cell 386 of oxygen concentration monitoring unit 354 while thegases exhaled by a patient into nasal canula 352 are being monitored.

At the same time, the pressure in sampling cell 388 is measured byabsolute pressure transducer 356 with the pressure value being comparedto the stored reference value. During oxygen concentration monitoringoperation of system 384, absolute pressure transducer 356 continuouslymonitors the pressure in sidestream line 437 with the current pressurevalues being compared with the stored value. This insures that system384 is operating within parameters which provide an accurate measurementof oxygen concentration by making it possible to almost instantaneouslyidentify problems which might effect the accuracy of the oxygenconcentration because such problems will affect the pressure in flowpath 437. By way of example only, such problems include a dislodgment ofnasal canula 352 and a partial blockage of sampling cell 388. Anocclusion alarm 447 on control unit 422 is activated if an occlusion isdetected.

Systems employing the principles of the present invention may beemployed in situations where the ambient pressure changes. For example,the system might be used to monitor a patient being transported byhelicopter to a medical facility. As the helicopter rises, the ambientpressure drops. By periodically checking the ambient pressure, one caninsure that the pressure in system 350 is compared to the currentambient baseline pressure, insuring that occlusions and other problemsare detected while false alarms are avoided. To this end, the clock 448shown in FIG. 25 periodically causes valve control 450 to shift valve424 to the NC position to obtain an updated reference pressure.

It is important in making an accurate oxygen concentration measurementof the gases flowing through sidestream sampling cell 388 have. Aconstant rate-of-flow through the sampling cell 388. Variations in theflow rate would cause inaccuracies in oxygen concentration measurementof the gases being monitored through the sampling cell. Flow ratevariations are detected by differential pressure transducer 358 whichapplies appropriate corrections to pump speed control 438. The pump isthereupon speeded up or slowed down to the extent necessary to keep theflow rate constant.

With pump 362 running, the system is zeroed for oxygen content bycirculating air from the ambient surroundings through system flow path437 until the system pressure stabilizes. With this accomplished, system350 is initialized and monitoring of the oxygen concentration in theexhalations of patient 368 can proceed.

During the determination of the patient's oxygen concentration, dampingchamber 364 and an orifice 446 in flow path 437 cooperate to dampen theunavoidable oscillations in the back pressure of vacuum pump 362. Thisminimizes variations in the pressure of gases flowing through sidestreamsampling line 437 and sampling cell 388, minimizing if not eliminatingthe adverse effects which such pressure variations might have on theaccuracy of the oxygen concentration indicative signal generated byoxygen concentration monitoring unit 354.

At the conclusion of the oxygen concentration monitoring process, poweris removed from system 350. The system is then readied for the nextprocedure by flushing the system and/or replacing sampling cell 388. Theoxygen concentration monitoring process can also be interrupted andsample cell replaced at any time that an occlusion in the cell isdetected.

The invention may be embodied in many forms without departing from thespirit or essential characteristics of the invention. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive. The scope of the invention isindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed:
 1. Oxygen monitoring apparatus, comprising: a flowcomponent including: a flow passage; and a luminescable compositionpositioned so as to be exposed to gases flowing through said flowpassage, luminescence of said luminescable composition being quenchedwhen said luminescable composition is exposed to oxygen; and atransducer removably securable to said flow component and including: aradiation source which emits at least a wavelength of electromagneticradiation capable of exciting said luminescable composition; and adetector configured to sense electromagnetic radiation of at least onewavelength emitted by said luminescable composition and to produce asignal indicative of an intensity of said at least one wavelengthemitted by said luminescable composition, said detector being oriented,upon securing said transducer to said flow component, to receiveelectromagnetic radiation emitted by said luminescable composition. 2.Oxygen monitoring apparatus as defined in claim 1 in which said flowcomponent comprises an airway adapter.
 3. Oxygen monitoring apparatus asdefined in claim 1 in which said flow component comprises a samplingcell.
 4. Oxygen monitoring apparatus as defined in claim 1, furthercomprising: a reference detector; and a beam splitter for dividing thelight propagated from said radiation source between said data detectorand said reference detector.
 5. Oxygen monitoring apparatus as definedin claim 1 in which said luminescable composition is embedded in aporous polymer.
 6. Oxygen concentration monitoring apparatus as definedin claim 1 wherein at least one window in said flow componentfacilitates the transmission of electromagnetic radiation from saidradiation source to said luminescable composition and from saidluminescable composition to said detector.
 7. Oxygen monitoringapparatus as defined in claim 1 in which said detector comprises aphotodiode.
 8. Oxygen monitoring apparatus as defined in claim 1,further comprising a filter adjacent said radiation source to preventexposure of said luminescable composition to radiation of at least somewavelengths of electromagnetic radiation emitted from said radiationsource.
 9. Oxygen monitoring apparatus as defined in claim 1 in whichsaid transducer comprises at least one platform for supporting saidradiation source and said detector.
 10. Oxygen monitoring apparatus asdefined in claim 1 in which said luminescable composition comprises aphosphorescent organometallic complex.
 11. Oxygen monitoring apparatusas defined in claim 1, further comprising a temperature sensorconfigured to sense a temperature of at least one of said heatercomponent, said thermal capacitor, and said luminescable composition.12. Oxygen monitoring apparatus as defined in claim 1 in which saidluminescable composition is positioned within said flow passage. 13.Oxygen monitoring apparatus as defined in claim 1 in which said signalindicative of said intensity of said at least one wavelength emitted bysaid luminescable composition is also indicative of a concentration ofoxygen in respiratory gas to which said luminescable composition isexposed.
 14. Oxygen monitoring apparatus as defined in claim 1 in whichsaid radiation source emits electomagnetic radiation of at leastwavelengths from about 300 nm to about 600 nm.
 15. Oxygen monitoringapparatus as defined in claim 1 in which said luminescable compositionabsorbs electromagnetic radiation of at least wavelengths from about 400nm to about 700 nm.
 16. Oxygen monitoring apparatus as defined in claim1 in which said luminescable composition emits electromagnetic radiationof at least wavelengths from about 500 nm to about 1,100 nm.
 17. Oxygenmonitoring apparatus as defined in claim 1 in which said radiationsource is configured to emit said electromagnetic radiation in a pulsedmanner.
 18. Oxygen monitoring apparatus as defined in claim 1 in whichsaid radiation source is oriented, upon securing said transducer to saidflow component, to emit said electromagnetic radiation toward saidluminescable composition.
 19. Oxygen monitoring apparatus as defined inclaim 1, wherein said at least one wavelength of electromagneticradiation emitted by said luminescable composition and sensed by saiddetector is in the visible light range.
 20. Oxygen monitoring apparatusas defined in claim 19, further comprising a filtering elementconfigured to prevent said detector from receiving wavelengths ofelectromagnetic radiation emitted from said luminescable compositionthat do not indicate an amount of oxygen to which said luminescablecomposition has been exposed.
 21. Oxygen monitoring apparatus as definedin claim 1 which comprises a second radiation source which emits atleast a calibration wavelength of electromagnetic radiation.
 22. Oxygenmonitoring apparatus as defined in claim 21 in which said secondradiation source comprises a light-emitting diode.
 23. Oxygen monitoringapparatus as defined in claim 21 in which said detector, upon sensing atleast said calibration wavelength, generates a calibration signal. 24.Oxygen monitoring apparatus as defined in claim 23 in which at least oneof said transducer and said flow component includes a resilientlydisplaceable member which is configured to be displaced upon assembly ofsaid transducer with said flow component so as to expose said firstmember of said temperature control component and said second member ofsaid temperature control component to one another.
 25. Oxygen monitoringapparatus as defined in claim 21 in which said electromagnetic radiationemitted from said second radiation source does not substantially causesaid luminescable composition to luminesce.
 26. Oxygen monitoringapparatus as defined in claim 21 in which said second radiation sourceemits at least an orange, red, or infrared wavelength of electromagneticradiation.
 27. Oxygen monitoring apparatus as defined in claim 1 inwhich said transducer includes a center section and first and second endsections on opposite sides of said center section which cooperate todefine a well configured to receive a portion of said flow component.28. Oxygen monitoring apparatus as defined in claim 27 in which saidradiation source is positioned at least partially in said first memberof said transducer and said detector is positioned at least partially insaid second member of said transducer.
 29. Oxygen monitoring apparatusas defined in claim 1 in which said radiaton source comprises alight-emitting diode.
 30. Oxygen monitoring apparatus as defined inclaim 29 in which said radiation source emits at least a blue or greenwavelength of visible light.
 31. Oxygen monitoring apparatus as definedin claim 1 in which said luminescable composition comprises afluorinated porphyrin.
 32. Oxygen monitoring apparatus as defined inclaim 31 in which the luminescable composition comprises at least one ofpalladium mesotetraphenyl porphine, platinum meso-tetraphenyl porphine,palladium meso-tetra (perfluoro) phenyl porphine, and platinummeso-tetra (perfluoro) porphine.
 33. Oxygen monitoring apparatus asdefined in claim 1 in which said detector comprises a photodiode. 34.Oxygen monitoring apparatus as defined in claim 33 in which saidphotodiode comprises a PIN silicon photodiode.
 35. Oxygen monitoringapparatus as defined in claim 1 in which said transducer is configuredto communicate said signal to a processor.
 36. Oxygen monitoringapparatus as defined in claim 35, wherein said processor is configuredto increase a signal-to-noise ratio of said signal.
 37. Oxygenmonitoring apparatus as defined in claim 35 in which said processorconverts said signal into an oxygen concentration signal.
 38. Oxygenmonitoring apparatus as defined in claim 37 in which said processorutilizes a first signal processing protocol if the oxygen concentrationin monitored gases is low and utilizes a second signal processingprotocol if the oxygen concentration in said monitored gases is high.39. Oxygen monitoring apparatus as defined in claim 1 in which a portionof said transducer is configured to removably interconnect with acomplementary portion of said flow component.
 40. Oxygen monitoringapparatus as defined in claim 39 in which said portion of saidtransducer receives said complementary portion of said flow component.41. Oxygen monitoring apparatus as defined in claim 39 in which at leastone of said flow component and said transducer is configured to preventimproper assembly of said flow component and said transducer.
 42. Oxygenmonitoring apparatus as defined in claim 1, further comprising atemperature control component configured to maintain said luminescablecomposition at a substantially constant temperature.
 43. Oxygenmonitoring apparatus as defined in claim 42 in which said temperaturecontrol component includes members in both said transducer and said flowcomponent.
 44. Oxygen monitoring apparatus as defined in claim 42 inwhich said flow component includes at least a portion of saidtemperature control component.
 45. Oxygen monitoring apparatus asdefined in claim 42 in which said transducer comprises at least aportion of said temperature control component.
 46. Oxygen monitoringapparatus as defined in claim 45 in which said filtering elementcomprises an optical filter positioned in an optical path between saidluminescable composition and said detector.
 47. Oxygen monitoringapparatus as defined in claim 42 in which said transducer includes afirst member of said temperature control component and said flowcomponent includes a second member of said temperature controlcomponent.
 48. Oxygen monitoring apparatus as defined in claim 42 inwhich said temperature control component includes: a heater componentcarried by said transducer; and a thermal capacitor carried by said flowcomponent and located proximate to said luminescable composition. 49.Oxygen monitoring apparatus as defined in claim 48 in which said heatercomponent protrudes through an aperture of said transducer.
 50. Oxygenmonitoring apparatus as defined in claim 48 in which said heatercomponent comprises a thermally conductive component and a thick filmheater affixed to said thermally conductive component.
 51. Oxygenmonitoring apparatus as defined in claim 48 in which said thermalcapacitor is in intimate contact with a matrix that carries saidluminescable composition.
 52. Oxygen monitoring apparatus as defined inclaim 51 in which said thermal capacitor is located within an apertureof the airway adapter and at least edge portions of said matrix are heldbetween said thermal capacitor and a surface of said aperture. 53.Oxygen monitoring apparatus as defined in claim 48 further comprising atemperature control associated with said heater component.
 54. Oxygenmonitoring apparatus as defined in claim 53 in which said temperaturecontrol comprises a PID controller.
 55. Oxygen monitoring apparatus asdefined in claim 48 in which, upon assembly of said flow component andsaid transducer, said heater component and said thermal capacitor arebiased against one another.
 56. Oxygen monitoring apparatus as definedin claim 55, further comprising a biasing member which, upon assembly ofsaid flow component and said transducer, biases said heater componentand said thermal capacitor against one another.
 57. Oxygen monitoringapparatus as defined in claim 56 in which said resilient biasing memberis configured to displace said heater component relative to a remainderof said transducer to compensate for misalignments between said heatercomponent and said thermal capacitor.
 58. Oxygen monitoring apparatus asdefined in claim 1 in which said luminescable composition communicateswith said flow passage by way of a sidestream conduit.
 59. Oxygenmonitoring apparatus as defined in claim 58, further comprising a pumpin communication with said sidestream conduit, said pump beingconfigured to effect a flow of one or more gases into contact with saidluminescable composition.
 60. Oxygen monitoring apparatus as defined inclaim 59, further comprising a pump controller for controlling pressurewithin said sampling cell.
 61. Oxygen monitoring apparatus as defined inclaim 60 in which said pump controller operates based on signals from apressure transducer in communication with said sampling cell.
 62. Oxygenmonitoring apparatus as defined in claim 59, further comprising anaccumulator in said flow path to dampen the pressure pulses generated byoperation of said pump.
 63. Oxygen monitoring apparatus as defined inclaim 62, further comprising a flow restrictor in said flow path tofurther dampen pressure pulses generated by operation of said pump. 64.Oxygen monitoring apparatus as defined in claim 59 in which saidluminescable composition is located within a sampling cell.
 65. Oxygenmonitoring apparatus as defined in claim 64 in which said sampling cellis replaceably removable from the system said flow component.
 66. Oxygenmonitoring apparatus as defined in claim 64, further comprising apressure transducer configured to compare the actual pressure in saidsampling cell with said baseline pressure.
 67. Oxygen monitoringapparatus as defined in claim 66, further comprising an alarm that isactivated if said pressure in said sampling cell varies from saidbaseline pressure.
 68. Oxygen monitoring apparatus as defined in claim66 in which said pressure transducer comprises: an absolute pressuretransducer; a differential pressure transducer; and a valve which isselectively operable to provide communication between said flow path andsaid absolute pressure transducer, said flow path and ambientsurroundings, and said flow path and said differential pressuretransducer.
 69. Oxygen monitoring apparatus as defined in claim 66,wherein said pressure transducer comprises a differential pressuretransducer and further including a flow restrictor in said flow path.70. Oxygen monitoring apparatus as defined in claim 64 in which saidsampling cell communicates with ambient surroundings to establishatmospheric pressure as a baseline pressure in said sampling cell. 71.Oxygen monitoring apparatus as defined in claim 1 in which saidluminescable composition is carried by a matrix.
 72. Oxygen monitoringapparatus as defined in claim 71 in which said matrix includes poreshaving sizes of about 0.1 μm to about 10 μm.
 73. Oxygen monitoringapparatus as defined in claim 71 in which said matrix has a thickness ofabout 5 μm to about 20 μm.
 74. Oxygen monitoring apparatus as defined inclaim 71 in which said matrix comprises a polymer.
 75. Oxygen monitoringapparatus as defined in claim 74 in which said polymer comprises atleast one of a silicone, a polycarbonate, a polystyrene, a polymethylmethacrylate, a polyvinyl chloride, a polypropylene, a polyester, and anacrylic copolymer.
 76. Oxygen monitoring apparatus as defined in claim75 in which said polymer is a track etched polycarbonate.
 77. Oxygenmonitoring apparatus as defined in claim 74 in which said polymer is ahydrophobic polymer.